Mercury is present in coal firing flue gas and heavy oil firing flue gas, or in the flue gas from refuse incineration power generation, and it is imperative that this mercury be removed prior to the flue gas being emitted into the atmosphere. Among the conventional technologies for removing mercury from flue gas is a method in which a solid absorbent such as activated carbon is installed in an initial stage of a stack and mercury is absorbed by the solid absorbent at a stage prior to the flue gas being emitted into the atmosphere, and a method in which a wet-type denitrification apparatus is provided on a flow path that extends from a boiler to a stack along which flue gas is made to flow, and a mercury oxidizing agent is placed on the upstream side of the wet-type denitrification apparatus, whereupon, after water-insoluble mercury contained in the flue gas has been converted into water-soluble mercury chloride and the like, it is absorbed by the wet-type denitrification apparatus. Moreover, another method exists in which an oxidizing agent is added to a wet-type denitrification absorption liquid, and mercury in the denitrification absorption liquid is oxidized and converted into water-soluble mercury.
Furthermore, in Patent Document 1 described below, technology is disclosed in which, in a flue gas processing apparatus that is provided with a denitrification apparatus and a wet-type denitrification apparatus, HCl or the like is fed to the upstream side of the denitrification apparatus so that mercury is oxidized by the denitrification apparatus. In Patent Document 2 described below, technology is disclosed in which flue gas is brought into contact with metal chloride or the like on the upstream side of a wet-type denitrification apparatus so that water-insoluble mercury is converted into water-soluble mercury chloride which is then absorbed by the wet-type denitrification apparatus. In Patent Document 3 described below, technology is disclosed in which flue gas is brought into contact with a carrier carrying a catalyst upstream from a wet-type denitrification apparatus so that water-insoluble mercury is converted into water-soluble mercury chloride which is then absorbed by the wet-type denitrification apparatus.    [Patent Document 1] Japanese Patent Application Laid-Open (JP-A) No. 10-230137    [Patent Document 2] Japanese Patent Application Laid-Open (JP-A) No. 2000-197811    [Patent Document 3] Japanese Patent Application Laid-Open (JP-A) No. 2003-053142
The following problems, however, exist in the above described conventional technology.
In the technology in which a solid absorbent is installed in an initial stage of a stack, for example, no efficient means has been established for processing the used solid absorbent so that there is a possibility that the environment will be affected. Moreover, because the structure in which a mercury oxidizing agent is provided on the upstream side of a wet-type denitrification apparatus and the structure in which an oxidizing agent is added to a wet-type denitrification absorption liquid are structures in which an oxidizing agent is injected directly into flue gas or waste water, there is a possibility that the environment will be affected. If a need arises to perform additional processing in order to reduce the effects on the environment, then the processing load increases. Moreover, in the structure in which an oxidizing agent is injected directly, equipment to inject the oxidizing agent is necessary so that there is an increase in equipment costs.
Furthermore, in the technology disclosed in Patent Document 1 as well, because a structure is employed in which a chemical substance such as HCl or the like is positively injected into the flue gas, there is a possibility that a need will arise to perform additional processing on the flue gas or waste water in order to reduce the effects on the environment. In this case, the processing load increases. In the technology disclosed in the above described Patent Document 2, there is the drawback that metal chloride, which is corrosive and toxic and is difficult to handle, must be used. In the technology disclosed in the above described Patent Document 3, because a structure is employed in which a catalyst that requires a carrier is produced, there is an increase in the production costs in order to produce the catalyst. Furthermore, because the structure requires that a mercury oxidizing apparatus be newly provided, there is also an increase in equipment costs.